Clarify U2F attestation verification

[apowers313]

Step 4 of the U2F attestation verification procedure could use some clarification:

Convert the COSE_KEY formatted credentialPublicKey (see Section 7 of [RFC8152]) to CTAP1/U2F public Key format [FIDO-CTAP].

Suggest adding "Section XYZ of [FIDO-CTAP]. CTAP is a big spec and it's not immediately clear that there's actually any section that defines a public key format.

• Let publicKeyU2F represent the result of the conversion operation and set its first byte to 0x04. Note: This signifies uncompressed ECC key format.
• Extract the value corresponding to the "-2" key (representing x coordinate) from credentialPublicKey, confirm its size to be of 32 bytes and concatenate it with publicKeyU2F. If size differs or "-2" key is not found, terminate this algorithm and return an appropriate error.
• Extract the value corresponding to the "-3" key (representing y coordinate) from credentialPublicKey, confirm its size to be of 32 bytes and concatenate it with publicKeyU2F. If size differs or "-3" key is not found, terminate this algorithm and return an appropriate error.

I think a final bullet should be added stating Let publicKeyU2F be the concatenation of (0x04 || x || y).

[emlun]

Thanks, it turns out the [FIDO-CTAP] reference was actually wrong and should instead be a reference to [FIDO-U2F-Message-Formats].

[yackermann]

@emlun maybe "1. Convert the COSE_KEY formatted |credentialPublicKey| (see Section 7 of [[!RFC8152]]) to Raw ANSI X9.62 public key format(see ALG_KEY_ECC_X962_RAW in Section 3.6.2 of [[!FIDO-Registry]]).

[emlun]

@herrjemand For the record, could you submit that as a review in PR #861 instead?

[emlun]

@herrjemand Bump

[yackermann]

@emlun Sorry have not seen you previous message *)

[yackermann]

@emlun Sorry have not seen you previous message *)